Photomasks are chrome patterned quartz templates (Refer to FIG. 1) employed in a photolithographic process (sometimes referred to as photoengraving) for transferring the integrated circuit designer's ideas onto silicon substrates. An integrated circuit is manufactured by succession transfers of unique photomask layers composing a device set of 15 to 25 layers onto silicon wafers coated with a photo-active polymer called a resist. The pattern transfer is accomplished using ultraviolet (UV: 365 nanometers) or deep-ultraviolet (DUV: 248 nanometers) exposure tools called wafer steppers that optically reduce the photomask image by factor of 4.times. or 5.times. its original size. As the wafer stepper exposes the silicon wafer, the stage of the wafer stepper moves in precise incremental steps to create a 2-dimensional matrix of identical patterns on the coated silicon wafer. The minute features represented in the resist define the areas for doping or interconnection and will ultimately constitute the functional elements (i.e., sources, drains, and gates) that make up the integrated circuit device. When the wafer manufacturing process is complete, the matrix of identical patterns will be cut into separate elements each yielding an integrated circuit chip.
Manufacturing specifications for photomasks are very tight with regards to critical dimension control (feature size), pattern placement (registration), hard defects (pattern errors) and contamination (soft defects), as any imperfections in the photomask will be replicated many times in the silicon and adversely impact the functional integrity of the resulting integrated circuits. Accordingly, photomask manufacturers invest considerable time in verification of product quality using an extensive line of sophisticated measurement and automated inspection tools. In additional, a safeguard is taken to ensure that when the product is shipped and throughout its lifetime of use by the IC manufacturer it is protected from the deposition of airborne contaminants. Referring again to FIG. 1, a dust cover called a frame-supported pellicle (pellicle) is mounted on the photomask (1). The frame-supported pellicle (2) is a transparent polymeric membrane (pellicle membrane) bonded in a taut drum-like manner to a rigid aluminum frame (3), also known as a pellicle-frame. The opposing side of the pellicle-frame, bonded to the photomask surface, is coated with a pressure-sensitive adhesive (4). The mounted frame-supported pellicle enshrouds and protects the important pattern data (5) of the photomask from the deposition of airbourne contaminants (6) yet is transparent to the actinic radiation of the wafer stepper tool. The pellicle frame height is an important functional element of the frame-supported pellicle. Its dimension is established such that any contaminants (7) falling on the pellicle membrane are out of the focal plane of the wafer stepper tool and unable to image on the resist coated silicon wafer. Each brand and model of wafer stepper has its own specifications for pellicle-frame height and size. The length and width of frame-supported pellicles vary based on the size of the stepper's exposure quality area, the size of the photomask substrate, and the amount of pattern area. In short, there is not one frame-supported pellicle size but rather an assortment of sizes. Regardless of size or type, all pellicles perform the same basic functional purpose of protecting the photomask from contamination.
Referring now to FIG. 2, is a summary of the photomask manufacturing process. As shown, frame-supported pellicle mounting (904) is one of the final steps in the photomask manufacturing process (800-905). It occurs, only after it has been verified that the photomask meets its specifications for critical dimension control (804), pattern placement accuracy (805), and is free of hard defects and contamination (900-902). Once the frame-supported pellicle has been mounted on the photomask, one final "through-the-pellicle" (through-pellicle) inspection (904) is performed to verify that no hard defects or contamination have been added to the photomask since the time of its last "pre-pellicle" inspection (900). If and only if, the photomask passes its through-pellicle inspection, can it then be shipped to the customer or used internally for IC manufacturing. It is also assumed, although not verified, that the mounted frame-supported pellicle has no distorting affect upon the photomask substrate that will compromise the accuracy of its pattern placement (registration).
In actuality the photomask frequently fails its final "through-pellicle inspection (904), an event that given the prior art of pellicle frame design results in the destructive removal of the frame-supported pellicle (1001) and sometimes destroys the photomask, itself. Furthermore, photomask distortions have been directly correlated to the application of the prior art's pellicle frame and have been measured as pattern placement errors.
The yield of the pellicle mounting process is defined as the percentage of photomasks which, though passing their pre-pellicle inspection, ultimately fail their final "through-pellicle" inspection and thereby require the removal of the frame-supported pellicle. The pellicle process is arguably the lowest yielding area in the photomask manufacturing process. Pellicle yields are frequently as low as 70% for tight specification product. The causes of yield loss are multifarious. Some of the most prevalent causes include: the automated inspection equipment is not 100% accurate. Defects or contamination can be missed during the pre-pellicle inspection that are latter captured during the final through-pellicle inspection. This is particularly prevalent when the photomask has a defect size specification approaching the limit of the detection capability of the inspection tool. Defects found during the pre-pellicle inspection may be inadequately repaired and later found during the final through-pellicle inspection. Defect inspection, contamination inspection, and pellicle mounting often take place in separate cleanroom bays by different production personnel. The considerable handling and transportation of the photomask risks the introduction of contaminants or damage to the product from scratches. The contamination inspection immediately preceding the pellicle mount process is always a blind process. There is no way of knowing if the photomask is free of contaminants until after its "through-pellicle" inspection since a post-clean pre-pellicle inspections itself risks introducing additional contamination. The pellicle mounting process is a labor-intensive manual operation very prone to the introduction of contaminants and susceptible to damaging the photomask and pellicle. What's more, the manual nature of the pellicle mount process is dependent on the skill of the pellicle mount operator and can vary dramatically from individual to individual. Furthermore, as photomasks specifications continue to tighten with regard to defects and contamination, these problems are sure to be exacerbated.
In the prior art of pellicle frame design, the frame-supported pellicle is permanently bonded to the photomask after its "pre-pellicle" inspection without any assurance it will pass its final "through-pellicle" inspection. This is a major flaw in the design of the pellicle process that has considerable logistical and financial consequences. When defects or contamination are found underneath the pellicle, the pellicle must be removed from the photomask to allow for the repair of those defects or the removal of that contamination (2000). In the prior art, pellicle removal is an aggressive process. The pressure sensitive adhesive is so tenacious that the pellicle must be mechanically pried from the from the photomask surface in a manner that is in striking contrast to the delicate handling employed in every other step of the photomask manufacturing process. The removal process always results in the destruction of the pellicle. Pellicles can cost more than $400 a piece. As the industry moves toward larger and larger substrates (preparations are underway for introducing 8" or 9" photomasks) pellicle sizes will increase accordingly with a corresponding increase in cost. Simultaneously, the wafer industry is moving towards E-line ultra-violet (EUV: 193 nanometers) wafer exposure tools that will require pellicles with advanced polymeric membranes transmissive at these wavelengths. The combination of increasing size and more advanced pellicle membrane technology will appreciably increase the cost of pellicles and the financial loss associated with the prior art's re-pelliclization process.
Another tremendous disadvantage of the prior art is the risk pellicle removal process poses to the photomask itself Once the pellicle has been dismounted from the photomask the photomask must be cleansed of any residual adhesive left behind by the frame. This process entails its own inherent risks. The additionally handling can result in catastrophic errors such as breakage or scratches on the photomask. The cleaning process used to remove the adhesive may cause damage to the minute features on the photomask or introduce unknown contaminants that cannot be removed. The financial loss of rejecting the photomask is considerable. The cost of a 6".times.6".times.0.250" photomask can range from $4000 for a binary photomask to $20,000 for a "state-of-the-art" phase-shift photomask. As in the case with pellicles, the cost of photomasks will increase enormously when 8" or 9" substrates are introduced and thus, the financial liability of the prior art' pellicle process.
An additional matter that compounds the risks of pellicle removal is that there are no assurances that with a second round of pellicle application the photomask will pass its final "through-pellicle" inspection. In fact, often the cycle of pellicle removal and reattachment must be performed several times before the photomask passes or is destroyed in the process. A simple probability calculation shows that if the pellicle yield were 80%, there yet remains a 6.4% chance that the second pellicle will also have to be removed and the entire process begun again. Every such cycle consumes expensive pellicles and endangers the underlying photomask.
In the preferred embodiment, the proposed invention employs a novel frame design that permits a two-step process for affixing the pellicle to the photomask surface. In the first step, the pellicle is temporarily affixed to the photomask using a vacuum bonding frame. The vacuum bonded pellicle is sufficient to enshroud and protect the photomask throughout its final "through-pellicle" inspection process. In the event that defects are found during the "through-pellicle" inspection, the pellicle can be removed without any damage to itself, the underlying photomask, or requiring any post-processing of the photomask. If and only if, the final inspection verifies the photomask is defect and contamination free, is the pellicle permanently bonded to the photomask using an adhesive coated locking ring assembly.
The photomask quartz substrate has the characteristics of extremely low thermal expansion, great mechanical rigidity, and flatness specifications of between 0.5 and 2.0 microns with a very high surface polish. These characteristics are basic functional requirements necessary to limit mechanical distortions in the photomask that could result in pattern placement errors or surface flaws that could degrade resist coating and pattern imaging. In the prior art, the pellicle frame is manufactured in a manner that is inconsistent with the above requirements and has been observed to induce distortions in the pellicle mounted photomask. The pellicle frame is currently composed of anodized aluminum manufactured using a mechanical milling process. In the course of machining, internal stresses are released in the material that result in physical distortions of the frame. An attempt is made by the pellicle frame manufacturer to limit these distortions by machining the frame from a single block of material and employing intermediate metal relaxation methods. Although some improvement is achieved, it is limited and accomplished at the expense of considerable waist of the constituent material. In addition, very little, if any, mechanical lapping is used to improve the quality of the pellicle frame's mounting service. As a result, photomask distortions have been directly correlated to the application of the pellicle frame and have been measured as pattern placement errors using registration tools such as the Leitz LMS 2000. The distortions are particularly apparent on photomasks of size 5".times.5".times.0.090" and less so, but still observable on 6".times.6".times.0.250" material.
In the preferred embodiment, this invention eliminates the distortion and improves the flatness of the pellicle frame by replacing aluminum as the constituent material with a polymer that is compatible with an injection molding process. A judiciously selected semi-crystaline polymer such as Polyetheretherkeytone (PEEK) can provide excellent mechanical properties such as stiffness, dimensional stability, chemical resistance, a variety of surface finishes together with superior processing characteristics. As will be discussed in the preferred embodiment of this invention, the injection molding process will allow greater flatness to be achieved in the pellicle frame reducing distortion of the underlying photomask and eliminating pattern placement errors.